1. Field of the Invention
The present invention relates to intrusion detectors and, more particularly, to such detectors having a surveillance zone below the detector (often termed a "look-down" facility).
2. Description of Prior Art
It has become more and more common for intrusion detectors to be designed as zoned devices, i.e. devices adapted to detect the presence, or movement, of an intruder in one or more designated areas (zones) within a monitored space. In general, the plurality of surveillance zones is obtained using optical arrangements including lenses and/or mirrors. See, for example, U.S. Pat. No. 3,703,718 (Berman); U.S. Pat. No. 4,364,030 (Rossin); U.S. Pat. No. 4,429,224 (Wagli et al); U.S. Pat. No. 4,442,359 (Lederer); U.S. Pat. No. 4,523,095 (Keller-Steinbach); U.S. Pat. No. 4,703,171 (Kahl et al); U.S. Pat. No. 4,873,469 (Young et al); British Patent Application 2178532 (Racal-Guardall (Scotland) Ltd.); and European Patent Application 0177130 (Matsushita Electric Works, Ltd.).
The majority of zoned detectors are designed to be ceiling or wall mounted. Early detectors sensed presence in zones and/or movement in or between zones in the same horizontal plane as the detector or in planes tilted downwards at a shallow angle. This means that the surveillance zones were relatively far away from the detector and that areas close to or directly underneath the detector were not covered. More recently, detectors have been designed to incorporate a look-down facility, i.e. to include one or more surveillance zones below or close to the detector, usually in addition to the normal forward/side zones. Examples of detection devices incorporating a look-down facility are described in U.S. Pat. No. 3,631,434 (Schwartz); U.S. Pat. No. 4,752,769 (Knaup et al); U.S. Pat. No. 4,841,284 (Biersdorff); U.S. Pat. No. 5,083,025 (Bloberg) and European patent application 0177130 (Maximal Electrical Engineers Limited).
In the prior art intrusion detectors having a look-down facility, the look-down zone(s) have been produced in various ways. One technique uses a curvilinear mirror located above the sensing element to reflect radiation received from below and to focus it onto the sensing element (U.S. Pat. No. 4,841,284). Another technique uses a lens below the sensing element to focus radiation received from below onto a plane mirror located above the sensing element; the mirror reflects the focussed radiation onto the sensing element (U.S. Pat. No. 5,083,025). Yet another technique makes additional use of a lens already involved in imaging the far zones, and uses it in combination with a mirror spaced above the lens, the mirror being either inside or outside the detector housing (U.S. Pat. Nos. 3,631,434, 4,752,769, EP-A-0219954).
All of the known techniques for providing a look-down facility have disadvantages. The curvilinear mirror approach provides a single look-down zone of narrow aperture. Techniques using combinations of a spaced mirror and lens can provide a plurality of look-down zones if the lens is a Fresnel lens. However, the lens is required to have a short focal length and, preferably, a wide aperture, which is difficult to achieve when using typical Fresnel lenses.